Method for detection of micrometric and sub-micrometric images by means of irradiation of a mask or of a biological specimen with ionizing radiation

ABSTRACT

Described herein is a method for detection of micrometric and sub-micrometric images with high resolution and high dynamics contrast, and suited for enabling microradiography, x-ray microscopy and the production of coloured configurations in dielectric materials. The method consists of radiation of energy comprised between 20 and 2000 eV for irradiating a mask or a biological specimen and use of a detector consisting of LiF in the form of a crystal or a film. The power of the radiation released in the LiF detector during exposure is preferably ≧10 mW/cm 3 .

TECHNICAL FIELD

The present invention relates to a method for detection of micrometricand sub-micrometric images by means of irradiation of a mask or of abiological specimen with ionizing radiation.

BACKGROUND ART

Known for some time is the use of ionizing radiation of energy comprisedbetween 20 and 2000 eV, generally referred to as “soft x-rays”, in thefield of microradiography and x-ray microscopy (Bollanti et al., IlNuovo Cimento 20D, pp. 1685-1701, 1998).

In said techniques, normal photographic-emulsion-based photographicplates prove ineffective on account of the excessive size of the grainsof the emulsion itself as compared to the micrometric andsub-micrometric details, which are to be observed on the specimen.

For this reason, as detectors specific photosensitive plastics referredto as “photoresists” are used, of which the most widely utilized isrepresented by PMMA.

Even though detectors of a photoresist type enable viewing of themicrometric and sub-micrometric details, the radiographic imagesobtained are characterized by poor dynamics of contrast. In fact, thephotoresist does not respond where the dose of energy is too low, and issaturated completely where the energy dose is too high. In technicalterms, it may be said, for example, that the photoresist of a PMMA typeenables 5-to-6-bit images to be obtained.

Another disadvantage regarding the use of the photoresist concerns theneed for developing the photoresist itself using certain substances,such as alcohol of an MIBK type, capable of removing the parts ofpolymer impinged upon by the radiation. Said operation of developmentcauses a loss of spatial definition of the image on account of theinevitable lateral corrosion of the polymer.

A different type of x-ray microscopy is projection microscopy, in whichthe image of the specimen is projected on the detector with highenlargement, for example approximately 1000 times, and as detector acharge-coupled device (CCD) is used. For this type of microscopy, plasmalasers do not have a sufficient mean power, and a powerful monochromaticsource is therefore required, such as a synchrotron-light source, withthe obvious drawbacks due in particular to the high costs and to theinevitable encumbrance that the use of the synchrotron involves.

Turning our attention to another area of the art, and in particular tothe area regarding the realization of micro-optical devices, there hasfor some time been felt the need to colour just some well-defined areas(hereinafter referred to as configurations) of a crystal or film ofhalogenide with the highest spatial definition possible. In this regard,it has for some time been known that ionizing radiation may give rise toa colouring of dielectric materials, for example LiF crystals, caused bythe formation of punctiform lattice defects which, on account of thecolouring produced, are called “colour centres”. Some colour centresemit an intense luminescence if excited by a radiation of wavelengthlower than that of emission.

It is moreover known that the density of the colour centres generated ina crystal is approximately proportional to the square root of the doseof radiation absorbed and, should the radiation itself have a powerhigher than a given threshold value, given the same dose, the aforesaiddensity of the colour centres increases in a way that is directlyproportional to the power (J. H. Schulman and W. D. Compton “ColorCenters in Solids”, Pergamon Press, 1962).

In order to obtain a well-defined high-resolution colouring, a not verypenetrative ionizing radiation has been used, such as low-energyelectron beams. Such a technique suffers from the drawback of presentingparticularly long execution times. In fact, depositing of the spacecharge of the electrons, in particular at the end of their path, entailsan extreme slowness in writing high-resolution configurations.

There was thus been felt the need to make available a method fordetection of high-definition micrometric and sub-micrometric images,together with high dynamics of contrast and particularly short executiontimes, which could find application both in techniques ofmicroradiography and x-ray microscopy and in the realization ofconfigurations in alkaline-halogenide crystals or films.

DISCLOSURE OF INVENTION

The present invention relates to a method for detection of micrometricand sub-micrometric images by means of irradiation of a mask or of abiological specimen with ionizing radiation, characterized in that saidionizing radiation has an energy comprised between 20 and 2000 eV, andin that it comprises a detector consisting of LiF designed to receivesaid ionizing radiation.

According to a preferred embodiment of the method of the presentinvention, said ionizing radiation releases in the detector a powerhigher than or equal to 10 mW/cm³.

Preferably, the method according to the present invention envisages thatsaid ionizing radiation will be generated by a plasma-laser system,which comprises a pulsed excimer laser and a target material.

Preferably, the method according to the present invention envisages thatthe detector will consist of a LiF film.

BRIEF DESCRIPTION OF THE DRAWINGS

The ensuing examples are to be considered non-limiting, and are providedwith a purely illustrative purpose for a better understanding of theinvention. In addition, reference will be made to the following figures:

FIG. 1 is a schematic illustration of a device for makingmicroradiographs or colour configurations in crystals or on films bymeans of irradiation of soft x-rays;

FIG. 2 is an image of luminescence of a LiF crystal treated with softx-rays according to the diagram of FIG. 1;

FIG. 3 is a detail of a wing of a dragon fly x-rayed on LiF (a) and onphotoresist made of PMMA (b); and

FIG. 4 is the radiograph of a polypropylene phantom having a thicknessof 0, 1, 2, 3 μm, obtained (according to the diagram designated by (a))on 2-μm thick LiF film (b) and on photoresist made of PMMA (c, d).

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES Example 1 MicrometricContact Detection of a Mask on a LiF Crystal

As illustrated schematically in FIG. 1, a device 1 was built for contactdetection of masks or biological specimens.

Specifically, the device of FIG. 1 comprises a 1J—10 ns XeCl excimerlaser designated as a whole by 2, a strip 3 of target material,preferably copper or iron, which, together with the excimer laser,constitutes the plasma-laser source, a LiF crystal or a LiF film 4 setat a distance of 10 cm from the plasma-laser source, and a mask 5 set incontact with the LiF crystal or the LiF film 4.

Using the device described above, microscopic detection of a grid havinga diameter of 3 mm and a pitch of 60 μm was performed. The LiF crystalwas treated with soft x-rays, the spectrum of emission of which wasconcentrated in the range of 20-200 eV (i.e., in the extremeultraviolet—EUV), and generated with 1000 pulses on a target made of Feusing the laser of FIG. 1 functioning at 1 Hz. The laser used may bemade to function at up to 100 Hz and is thus able to emit 1000 pulses in10 seconds.

Given the extremely high absorption in the EUV, each shot deposits inthe crystal a dose of 30 J/cm³, with a corresponding power of 3 GW/cm³.

In LiF specimens, the 1000 pulses were sufficient to generate an intenseluminescence in the spectral range of the visible (green, yellow, red)by excitation with blue light.

FIG. 2 presents the corresponding image of luminescence observed with anoptical microscope having a 40× lens. From FIG. 2 there clearly emergesthe intense luminescence of the areas not shielded from the x-rays andthe absence of luminescence in the areas in which the grid shaded thex-rays. In addition, there was obtained a transverse resolution of lessthan 1 μm, and the precision of the measurement was limited by theproperties of the optical microscope used for obtaining the image ofluminescence.

The lack of uniformity of the luminescence in FIG. 2 is due to theincomplete uniformity of the blue light generated by an Ar laser usedfor excitation of the luminescence and not by lack of uniformity in thedensity of the optically active colour centres.

From the example given above, it emerges clearly how, with the methodaccording to the invention, it is possible to build micro-opticaldevices with a high resolution, over vast areas, and in times at least100 times shorter than those afforded by electron-beam irradiation.

Example 2 Microradiography of a Biological Specimen on a LiF Film and ona Photoresist

Using the device illustrated in FIG. 1 and replacing the mask with abiological specimen, microradiography was performed of the wing of adragon fly obtained on a LiF film and, for comparison, microradiographywas performed of the other wing of the same insect on a photoresist madeof PMMA. The two exposures were obtained simultaneously in the sameexperimental conditions by means of 1100 pulses on a target made ofcopper set at a distance of 10 cm from the biological specimen.

FIG. 3 presents the corresponding microradiographs observed at anoptical microscope with 20× lens. In particular, the microradiography onPMMA was made with an operation of development for one minute in MIBK,and the observation at the atomic-force microscope (AFM) of the samePMMA did not enable images better than those presented in FIG. 3 to beobtained. From the comparison of the two microradiographs, it is evidenthow the peculiar performance of response of LiF enables a quality ofimage considerably higher than the one obtained on the photoresist. Infact, PMMA allows the internal structure of the wing to be only scarcelydiscerned and, furthermore, it is more vulnerable to the bombardment ofthe considerable amount of detritus emitted by the plasma-laser source,which is made up of particles of melted metal having a diameter rangingtypically between 0.1 and 100 μm.

In particular, for the LiF film the dynamics of the image (number ofshades of grey and hence number of bits of the image) is not limited bythe characteristics of corrosion of the photoresist, which requires amechanical reading of the information following upon development, butprincipally by the efficiency of formation of the aggregated luminescentcentres in the LiF film in the conditions of dose and of power used inthe x-ray irradiation.

Example 3 Microradiography of a Ploypropylene Phantom of VaryingThickness on a LiF Film and on a Photoresist

Using the device illustrated in FIG. 1, radiography was performed on apolypropylene phantom having a thickness of 0, 1, 2, 3 μm on a LiF filmand, for comparison, on a photoresist made of PMMA.

The radiograph was obtained by means of 1100 pulses on a target made ofcopper set at a distance of 10 cm from the polypropylene phantom. Thefluences that reached the detector made of LiF or the detector made ofPMMA in the different areas corresponding to the different thicknessesof the phantom were, respectively, 600 mJ/cm², 4 mJ/cm², 2 mJ/cm², and 1mJ/cm², as indicated in the quadrant (a) of FIG. 4.

From the comparison of the quadrants (b), (c) and (d) of FIG. 4, it maybe noted that on the LiF detector (quadrant (b)) there are readilyrecognizable the “impressions” of x-ray radiation corresponding to allthe different values of dose, and that on the PMMA (quadrants (c) and(d)) only the area with direct exposure corresponding to a thickness of0 μm is clear, whereas the others may be easily confused with oneanother, even if analysed at the atomic-force microscope.

In the light of Example 1, it is evident how the method according to thepresent invention enables very high-resolution configurations to beobtained and in a length of time at least 100 times shorter than thatrequired, given the same resolution, by techniques with irradiationusing electron beams.

Said advantages can be directly connected to the possibility ofirradiating, at the same moment, an extended portion of mask.

In the light of Examples 2 and 3, it is evident how the method accordingto the present invention enables micrometric and sub-micrometric imagesto be obtained having a definition and a dynamics of contrast that areclearly higher than those obtained using the methods of the known art.

It is, in fact, important to emphasize both that the transverseresolution is not limited by the grain of the photographic emulsion,which is of the order of a few micron, and that the dynamics is notlimited by the properties of development of the negative and henceenables 8 to 12 bit images to be obtained unlike the 5 to 6 bit imagesobtained using the PMMA photoresist.

It has been found that the aforesaid advantages are more evident if theLiF is used in the form of a film instead of a crystal. The performanceof the LiF film as detector may be continuously improved byappropriately controlling the parameters of growth of the film itself,which, in turn, influence the morphological, structural and opticalcharacteristics of the material deposited.

As compared to the known art regarding the techniques ofmicroradiography or x-ray microscopy, another important advantage of themethod according to the present invention derives from the fact that thereading of the detector takes place by luminescence, i.e., by means of atechnique by several orders of magnitude more sensitive than any othersystem that is based upon absorption, such as photographic plates orphotoresists.

The LiF crystal or LiF film may be read using an optical microscope or aconfocal and/or near-field microscope (SNOM—scanning near-field opticalmicroscope). As is obvious, using the method according to the presentinvention no operation of development is envisaged, as occurs, instead,in the case where the detector is a photoresist, and this constitutes anadvantage both for reasons linked to the resolution and for reasonslinked to the praticality of the method itself.

In addition, it is important to note that both the short duration of theapplication and the high dynamics of contrast may be further optimizedby increasing the power of the ionizing radiation.

Finally, a possible modification of the device illustrated in FIG. 1consists in the use of multilayer mirrors for reproducing in projectionthe mask or the biological specimen on the LiF crystal or on the LiFfilm. In this case, the transverse resolution is no longer limited bythe effects of diffraction that occur in the space between the mask orbiological specimen and the LiF detector. This enables a reduction ofthe resolution to values lower than 100 nm, but requires exclusive useof the energies of the x-rays that can be reflected effectively by themultilayer mirrors for x-rays (typically 90 eV), the estimatedpenetration of which (approximately 30 nm) is compatible with thethicknesses of LiF film.

The method according to the present invention finds particularapplication for carrying-out x-ray microscopy and

1. A method for detection of micrometric and sub-micrometric images bymeans of irradiation of a mask or of a biological specimen with ionizingradiation, characterized in that said ionizing radiation has an energycomprised between 20 and 2000 eV, and in that it comprises a detectorconsisting of LiF designed to receive said ionizing radiation.
 2. Themethod according to claim 1, characterized in that said ionizingradiation deposits on said detector a power ≧10 mW/cm³.
 3. The methodaccording to claim 1 or claim 2, characterized in that said ionizingradiation is generated by a plasma-laser system.
 4. The method accordingto claim 3, characterized in that said plasma-laser system comprises apulsed excimer laser and a strip of target material.
 5. The methodaccording to claim 4, characterized in that said pulsed excimer laser isan XeCl laser.
 6. The method according to any one of the precedingclaims, characterized in that said detector is a LiF film.
 7. The methodaccording to any one of the preceding claims, characterized in that saidmask or said biological specimen is set in contact with said LiFdetector.
 8. The method according to any one of claims 1 to 6,characterized in that it uses multilayer mirrors designed to reproducein projection said mask or said biological specimen on said detector. 9.A device for detection of micrometric and sub-micrometric images forirradiation of a mask or of a biological material with ionizingradiation, characterized in that it uses a method according to any oneof the preceding claims.
 10. The device according to claim 9,characterized in that it enables micro-radiography or x-ray microscopy.11. The device according to claim 9, characterized in that it enablesconfigurations for optical devices to be obtained.